AR Build

So I am putting together a AR and want to do a flat top. What is the difference between a A3 and A4 flat top?

Technically there is no such animal as an A3 upper receiver, and if there was it wouldn't be a flat top. An M16A3 is just a full auto version of an M16A2. Both have the exact same carry handle upper receiver.

With that said, some manufacturers call flat top uppers A3 for some reason that I've never understood. Marketing I guess.
 
I don't believe this to be necessarily true. The A3 has been used for a long time by Colt to designate flat-top AR-15s (AR-15A3), even before the adoption of flat-top rifles as standard by the military. You need to distinguish between military designations and civilian ones.
 
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John
413 579 1994
 
This thread is probably causing some confusion.. Some slight clarification -- (The following is military) The M16a3 was NOT flat-top. The M16a4 WAS flattop.

That being said...
CIVILIAN -- There was an A1 and an A2. Neither were flat top. Flattops came to the civilian market and many marketed them as "A3 Uppers".

Conclusion: An A3 Flat-Top is pretty much a re-named/marketed M16A4 (Military) upper for the intents and purposes of this convo.

Some manufacturers use A3 to designate a flattop that's not mil-spec but still a flattop and use a4 for the mil-spec a4 flattops. This is a convo that could go on for a long time really, for all intents and purposes with your build, an A3 is basically identical to an A4, just know what you're getting into whether it's mil-spec or billeted / slabside etc..

For what it's worth: vltor probably makes the best mainstream upper with their mur-1 if you've got a limitless budget with this build, otherwise, I'd save the cash and put the saved money into a better barrel.

Everyone's got their opinions on this stuff I guess, the vltor comment is just mine -- just mentioning this to avoid a dispute.
 
some companies have spec'd A4 upper receivers as receivers with M4 feed ramps. where as an A3 upper receiver does not have M4 feed ramps.
 
Now a lower question. I have a lower stamped multi cal.

I see companies that make one lower but have different caliber uppers, ie, .556, .22, 6.8, 5.45x39, etc. Can a "standard" multi cal lower be used on the complete uppers of these different calibers?
 
I don't believe this to be necessarily true. The A3 has been used for a long time by Colt to designate flat-top AR-15s (AR-15A3), even before the adoption of flat-top rifles as standard by the military. You need to distinguish between military designations and civilian ones.

Right, but my point was that an A3 flat top was never a military thing. Some individual companies may have marketed flattops as A3 rifles, but that was purely marketing and not an official military standard or any kind. In a nutshell, an A3 flat top upper could be anything that the company in question wanted to call an A3, and one company's version wouldn't necessarily be the same as the next company's.

Now a lower question. I have a lower stamped multi cal.

I see companies that make one lower but have different caliber uppers, ie, .556, .22, 6.8, 5.45x39, etc. Can a "standard" multi cal lower be used on the complete uppers of these different calibers?

I assume the caliber markings are only there to comply with some sort of federal law. There are a ton of calibers other than 5.56 that will work in an AR. Basically, if the round is short enough to fit through the mag well someone has probably built an upper for it at some point. (Yes, you can build one lower and use it with multiple uppers. For example I have a service rifle in .223 with an identical clone upper in .22LR for practice.)
 
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Yeah, the only purpose of "Cal-Multi" is for registration purposes -- instead of it being registered as a 5.56 you can register it as a .338 Lapua or something if you have a .338 upper receiver planned, etc. It doesn't effect the rifle in any way shape or form by itself.

There are some cal-multi lowers for a high premium that are better suited for larger calibers with slight modifications to them but unless you want to spend $200+ on a stripped lower not even including a LPK, you won't get into that stuff.
 
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